Color display device in which the area of a spherical lens equals the area of a set of RGB sub-pixels

ABSTRACT

The invention relates to a color display device comprising a chromatic dispersive grating and an active matrix possibly of liquid crystal type. This device moreover comprises a matrix of spherical microlenses, the shape of the cross section of the microlenses being such that it makes it possible to compensate for the spreading of the R G B colored beams by virtue of the use of a dispersive grating, and hence to make best use of the circular aperture of projection objectives.

BACKGROUND OF THE INVENTION

The invention relates to a color display device and more particularly toa liquid crystal color display device.

DISCUSSION OF BACKGROUND

In order to generate video images of large dimensions, there iscurrently a trend towards the use of active liquid crystal matrices(LCD) in projection devices.

The projection of color images can be carried out either on the basis ofthree active matrices, each illuminated by one of the primarycomponents, red, green or blue (R, G, B) or on the basis of a singlethree-color active matrix, in this case equipped with colored filters.

The incorporation of R, G, B colored filters into the structure of thescreen (LCD) makes it possible to produce color image projection devicesof simple design: monovalve projector since they consist of a source anda single LCD screen.

However, the presence of these colored filters leads to a low luminousyield related to the absorption and spatial distribution of thesecolored filters which divides the effective transmission of the LCDscreen by three.

The cost of such a screen also remains high since it is necessary toimplement extra masking levels to obtain these colored filters and toseek to increase the density of the image elements (pixels) required inthe case of a color screen (as compared with a monochrome screen) if itis wished to produce them with reasonable dimensions (diagonal less than6 inches).

To obviate these drawbacks, display screens are envisaged in which thecolored filters are replaced by a diffracting grating capable, on thebasis of a single white source, of dispersing the red, green, blue (R,G, B) chromatic beams in three different directions. It thus becomespossible, with the aid of arrays of lenses enabling the light of eachchromatic range to be focused substantially on a pixel of the screen, toproduce a three-color screen without colored filters, compatible withmonovalve projection. Each lens corresponds with a dot, representativeof an (R, G, B) triple of sub-pixels. FIG. 1 illustrates a displaydevice according to this prior art. A source (S) of a certain extentgenerates white light comprising the chromatic components (R, G, B).These components are together sent via a collimator to the dispersivegrating (RC) which then generates differently directed beams (R, G, B).An array of lenses (MLC) enables these beams to be focused on pixels ofthe active matrix (LCD). The spots (SR, SG, SB) are representative ofthe red, green and blue images of the source (S) in the various pixelswhich thus enable all of the light flux issuing from the source (S) tobe concentrated in the active matrix. At the exit of the matrix, a fieldlens (L) makes it possible to converge the set of colored beams in aprojection optic (PO). However, the chromatic dispersion used in thistype of device generates a horizontal spreading of the beams, which thenrequires an increased aperture of the projection optic horizontallywhereas the vertical aperture of this optic is not utilized. FIG. 2illustrates this dispersion. For a lens whose cross section hasdimensions h_(L) and i_(L), if p represents the spacing separating twocenters of sub-pixels, a horizontal spreading by p on each side of theimage from the lens is obtained.

In order to make best use of the conventionally circular aperture ofprojection objectives, the invention proposes a display device whichmakes it possible to provide the best solution to this problem of theunsuitability of using a dispersive chromatic array coupled with a setof triples of sub-pixels with the geometry of the projection optics.

SUMMARY OF THE INVENTION

More precisely, the subject of the invention is a color display devicecomprising a light source (S) simultaneously emitting several ranges ofchromatic components (R, G, B) and a spatial light modulator (LCD)comprising a set of triples of sub-pixels aligned along an axis x, eachtriple having a dimension sp_(x) along this axis x, each sub-pixelcorresponding to a chromatic component to be displayed, characterized inthat it also includes between the light source (S) and the spatial lightmodulator (LCD);

at least one chromatic separator (RC) angularly separating alongdifferent directions the light of the various chromatic ranges (R, G,B);

an array of spherical lenses (MLC) enabling the light of each chromaticrange to be focused substantially on a sub-pixel, the said array oflenses having dimensions substantially equal to those of the spatiallight modulator and each lens having a cross section defined by thedimensions Px and Py if ly is the dimension along an axis yperpendicular to the axis x, the axes x and y corresponding to the axesof the plane (Px, Py) defined by the spatial light modulator, the saidcross section having the same area as that of a triple but the dimensionlx of which along the axis x is smaller than the dimension 3 sp_(x).

In the device according to the invention, the light modulator canadvantageously be a liquid crystal screen. The array of spherical lenses(MLC) can be affixed to the spatial light modulator (LCD).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and other advantages will emergefrom reading the description which follows given by way of non-limitingexample, and the appended figures in which:

FIG. 1 illustrates an exemplary display device according to the priorart, using a diffracting grating;

FIG. 2 shows diagrammatically the spreading of the light beams, in thevicinity of a projection optic in a display device such as thatillustrated in FIG. 1;

FIGS. 3a and 3b illustrate exemplary arrangements of the image elementsof a liquid crystal screen;

FIG. 4 shows diagrammatically the superposition of a lens array on a"DELTA" structure in an exemplary display device according to theinvention;

FIG. 5 illustrates the superposition of the images from a lens which aremade by the illumination of the three beams, red, green, blue in anexemplary device according to the invention,

It is recalled that in display devices which use an active matrix (LCD),a set of three sub-pixels (R, G, B) makes it possible to define a whitepixel (D), and this set is generally called a dot. FIGS. 3a and 3b thusgive exemplary distributions of colors within a dot. FIG. 3a representsan arrangement in which the pixels of one and the same color are alignedalong a vertical direction ("stripe" configuration). FIG. 3b representsan arrangement in which the pixels of one and the same color are alignedalong an inclined direction but do not touch one another. In thisconfiguration, also called a "DELTA" structure, the elementary triplesare laid out in quincunx fashion and two rows are used to define aseries of white points, unlike the configuration described in FIG. 3a inwhich one row corresponds to a series of white points.

This "DELTA" configuration is particularly beneficial insofar as, forone and the same area of active matrix, it makes it possible to use N/2white points, if N is the number of white points used in a "stripe" typeconfiguration, to obtain the same image resolution as illustrated bycomparing FIGS. 3a and 3b. Moreover, the so-called "DELTA" structureleads to elementary sub-pixels whose geometry is better adapted to thatof the circular image of the source. Indeed, to obtain the same row-wiseand column-wise spatial resolution, a "stripe" pixel should be square,of side ps. This implies elementary pixel dimensions ps and ps/3.

In the "DELTA" structure such as represented in FIG. 3b, the column-wisespatial frequency identical to that corresponding to a "stripe"structure, corresponds to an elementary sub-pixel dimension h whereasthe row-wise spatial frequency corresponds to 3/2 l (l the otherdimension of the sub-pixel). It is then apparent that the elementaryarea of a "stripe" sub-pixel is equal to ps×ps/3, whereas in the "DELTA"structure this elementary area h×l=ps×3/2 ps. In this latter structure,elementary sub-pixels are employed which are of larger area and aboveall are better adapted to the circular geometry of the image of thesource which it is sought to focus in each elementary sub-pixel.

The general principle of operation of the device according to theinvention can be described accurately on the basis of FIG. 1.

The function of the separator (RC) is to angularly separate the threespectral components of the collimated light source (S). The threespectral bands are next focused along three axes in the focal plane ofeach lens. In the example shown diagrammatically in FIG. 1, thegreen-colored beam (G) is normal to the focal plane of the lenses (MLC)and the red (R) and blue (B) beams are symmetrical with respect to thisnormal and make an angle of incidence θi. Consequently, by adapting onthe one hand the angle of incidence θi of each spectral component on thespherical lenses and on the other hand the focal length f of theselenses it is possible simultaneously to achieve:

the successive focusing of each color in the plane of the active matrix(LCD) and centered on each row of sub-pixels addressed by the samecolor. The characteristics (θi, f) of the device depend on the size ofthe active matrix (LCD), on the number of pixels and on the distributionof the sub-pixels within a dot;

a gain in the transmission factor of the matrix (LCD); when the focusedcolored band to a dimension smaller than that of the elementarysub-pixel. The value of this gain depends on the following parameters,the aperture of the spherical lens, the geometrical extent of the lightsource (E_(s)) the size of the matrix (LCD) characterized by itsdiagonal (D).

The separation (RC) function can advantageously be provided by adiffractive grating whose spacing is perpendicular to the direction ofalignment of the sub-pixels R, G, B.

Herein is proposed an exemplary embodiment of the device adapted to asize of LCD screen possessing R, G, B sub-pixels distributed as trios.

The characteristics of the LCD screen are as follows:

a 16/9 format, a 4-inch diagonal, an image defined by 480 lines and 640columns, for an 80×60 mm² cell and ##EQU1## elementary sub-pixels if thedimensions of three R, G, B sub-pixels are ##EQU2## According to theconventional LCD technologies, the line width required for theaddressing of the pixels is around 15 μm (a line which masks part of thetransparent pixel);

the microlens array can be made directly on one of the backing plates ofthe LCD screen and be such as that represented in FIG. 4.

By using a light source S of geometrical extent E_(S) =80 mm² -sr, weshall compare the performances obtained in terms of the light fluxcapable of passing through a given projection optic.

In fact, the limit value of the geometrical extent E_(S) (max) of thesource depends on the size of the LCD screen. It may be expressed asfollows as a function of the various parameters of the device

    E.sub.S (max)=400 D.sup.2 ×(φ.sub.f.sup.2 /f.sup.2

where D is the diagonal of the screen expressed in inches

f the focal length of the lenses

φ_(f) : the useful width of the pixel (pixel minus the line width)

By considering the image of the source in the focal plane of the lens tobe of dimension equal to the useful width of a sub-pixel and hence thatE_(S) =E_(S) (max), it is possible to determine f and hence theextension t within the glass t=f.n, this extension t corresponding tothe distance between the matrix of lenses and the liquid crystal cell.

It is thus possible by calculating the focal length f, to determine theaperture of the projection optic N required to collect all the R, G, Bsub-pixel images defined by the array of lenses, taking into account thespreading of the said images.

Indeed, it is possible to calculate the aperture N of the objectiverequired to collect the three light fluxes R G B. Since N=t_(objective)/φ objective with t the extension of the objective of projective and φobjective the diameter illustrated in FIG. 1. Also, by homothety N=t/φwith φ representing the magnitude of the spreading of the threeilluminations R, G, B (illustrated in FIG. 5). This diameter φcorresponds to the diagonal of the rectangle defined by the dimensions250 μm×(80+125+83) μm. Or again 385 μm.

These calculations were carried out in three illustrative cases so as tocompare the device according to the invention with other types ofdevices according to the prior art. The results are gathered together inthe Table (I) below.

    ______________________________________                                                  1st case 2nd case   3rd case                                                  prior art                                                                              prior art  invention                                       ______________________________________                                        configuration                                                                             stripe     delta      delta                                       mask        15 μm   15 μm   15 μm                                    diagonal    4 inches   4 inches   4 inches                                    number of points                                                                          480 × 640                                                                          480 × 320                                                                          480 × 320                             dimensions of the                                                                         125 × 125                                                                          125 × 250                                                                          125 × 250                             white pixel                                                                   dimensions of a sub-                                                                      125 × 125/3                                                                        125 × 250/3                                                                        125 × 250/3                           pixel                                                                         dimensions of a lens                                                                      125 × 125                                                                          125 × 250                                                                          125 × 250                             dimensions of a lens                                                                      125 × 125                                                                          125 × 250                                                                          125 × 250                             orientation of the                                                                        homothetic homothetic oriented at                                 lenses/pixels                     90°                                  extension t 3.60 μm 900 μm  900 μm                                   aperture N  0.8        1.2        1.5                                         ______________________________________                                    

Illustrative case No. 1 results in an unrealistic type of lens andunrealistic type of objective (lenses on a 300 μm backing plate,difficult to produce). The flux of this type of projector is thenconstrained to be appreciably restricted.

Illustrative case No. 2 certainly leads to a realistic type of lens (900μm backing plate) but a highly apertured objective which is difficult toenvisage. In this case it is necessary to choose lenses with a largerextension so as to increase the aperture N. The projected flux is thenreduced.

In the illustrative case of the invention, altogether realistic valuesof extension and aperture are determined allowing a large luminous fluxto pass.

Indeed, it is possible to pass the flux from a 150 W lamp, of extentE_(S) =80 mm² sr, efficiently into a 4" cell, using microlenses on a 900μm backing plate. An exit flux of the order of 300 to 400 lumens is thenobtained from an initial flux of 12000 lumens, taking into account theconventional efficiency of an active liquid crystal matrix (5%) and ofthe area ratio of the cell to a circumscribed circle (60%).

Table (II) herein gives the performances obtained in cases 1, 2 and 3with the same realistic projection aperture (N=1.5), and allowscomparison with an active matrix using colored filters.

    ______________________________________                                                            Stripe +  delta + delta +                                          Colored Filters                                                                          homothetic                                                                              homothetic                                                                            optimized                               Configuration                                                                          no lens    lenses    lenses  lenses                                  ______________________________________                                        extension (μm)                                                                      none       900       1150    900                                     flux (lumens)                                                                          50          55        220    350                                     ______________________________________                                    

We claim:
 1. Color display device comprising a light sourcesimultaneously emitting several ranges of chromatic components and aspatial light modulator comprising a set of triples of sub-pixelsaligned along an axis x, each sub-pixel having a dimension sp_(x) alongthis axis x, and each sub-pixel corresponding to a chromatic componentto be displayed, said device further including between the light sourceand the spatial light modulator;at least one chromatic separatorangularly separating along different directions the light of the variouschromatic ranges; an array of spherical lenses enabling the light ofeach chromatic range to be focused substantially on a sub-pixel, thesaid array of lenses having dimensions substantially equal to those ofthe spatial light modulator and each lens having a cross section definedby the dimensions lx and ly with ly being a dimension along an axis yperpendicular to the axis x, the axes x and y corresponding to the axesof a plane (Px, Py) defined by the spatial light modulator, said crosssection having the same area as that of said set of triples and whereinsaid dimension lx along the axis x is smaller than 3 sp_(x).
 2. Colordisplay device according to claim 1, characterized in that thesub-pixels of one and the same color are aligned along a directioninclined with respect to the axis x, and are not adjacent and that thecenter of each lens corresponds substantially to the center of asub-pixel of one and the same color.
 3. Color display device accordingto claim 2, characterized in that the cross section of the lenses is arectangle identical to that defined by each set of triples, but orientedperpendicularly to the said set of triples in the plane.
 4. Colordisplay device according to claim 2, characterized in that the chromaticseparator includes a phase microstructure component obtained by opticalrecording in a photosensitive material.
 5. Color display deviceaccording to claim 2, characterized in that the spatial light modulatoris a liquid crystal screen.
 6. Color display device according to claim2, characterized in that the lens array is affixed to the spatial lightmodulator.
 7. Color display device according to claim 1, characterizedin that the cross section of the lenses is a rectangle identical to thatdefined by each set of triples, but oriented perpendicularly to the saidset of triples in the plane.
 8. Color display device according to claim7, characterized in that the chromatic separator includes a phasemicrostructure component obtained by optical recording in aphotosensitive material.
 9. Color display device according to claim 7,characterized in that the spatial light modulator is a liquid crystalscreen.
 10. Color display device according to claim 7, characterized inthat the lens array is affixed to the spatial light modulator.
 11. Colordisplay device according to claim 1, characterized in that the chromaticseparator includes a phase microstructure component a obtained byoptical recording in photosensitive material.
 12. Color display deviceaccording to claim 11, characterized in that the spatial light modulatoris a liquid crystal screen.
 13. Color display device according to claim11, characterized in that the lens array is affixed to the spatial lightmodulator.
 14. Color display device according to claim 1, characterizedin that the spatial light modulator is a liquid crystal screen. 15.Color display device according to claim 14, characterized in that thelens array is affixed to the spatial light modulator.
 16. Color displaydevice according to claim 1, characterized in that the lens array isaffixed to the spatial light modulator.